The present invention relates to a method and system for observing a specimen using a scanning electron microscope which serve to observe defects etc. occurring in a manufacturing process of a semiconductor wafer, a liquid crystal panel, or the like.
To increase the production yield of a semiconductor, early determination of a cause of occurrence of defects in a manufacturing process is important. At present, in semiconductor manufacturing places, defects are analyzed by using a defect inspection apparatus and an observing apparatus. The defect inspection apparatus is an apparatus which observes a wafer using an optical means or an electron beam and outputs detected defect coordinates.
It is important for the defect inspection apparatus to process a wide area at high speed. Therefore, in the defect inspection apparatus, the amount of image data is reduced by setting the pixel size (dimensions on a specimen that is detected by one pixel of a detector) of an image to be acquired as large (low in resolution) as possible. In many cases, even if presence of a defect is recognized from a detected low-resolution image, a detailed type of the defect cannot be determined. This is the reason why the observing apparatus is used. The observing apparatus is an apparatus which images a defect coordinates position on a wafer at a high resolution using an output of the defect inspection apparatus and outputs a resulting image.
To observe a defect in detail, a resolution on the order of several nanometers is necessary partly because the degree of miniaturization has increased in semiconductor manufacturing processes and accordingly defect sizes have decreased to of the order of tens of nanometers. Therefore, in recent years, observing apparatus (hereinafter referred to as review SEMs) using a scanning electron microscope (SEM) have come to be used widely. In semiconductor mass-production lines, observation work is desired to be automated. And review SEMs incorporate an ADR (automatic defect review) function of automatically collecting images of defect coordinates positions on a wafer and an ADC (automatic defect classification) function of automatically classifying the acquired images.
The depth of focus of scanning electron microscopes is about 0.5 to 1.0 μm. Therefore, to take an unblurred image, it is necessary to set the focusing position of an electron beam at a target focusing position. The term “target focusing position” means a focusing position that is located at a subject surface of a specimen and hence enables taking of an unblurred image. In general, a specimen has a variation in height and hence individual observation regions have different target focusing positions.
Auto focus is a function of automatically calculating a target focusing position of an observation region. One auto focus technique is such that plural images are taken while the focusing position is varied, a focus measure indicating the degree of focusing is calculated from each image, and a focusing position that provides a maximum focus measure is determined as a target focusing position. In an image that is taken with the focusing position set at a target focusing position, the density value of an edge portion varies steeply. On the other hand, in an image that is taken with the focusing position deviated from a target focusing position, the density value of an edge portion varies gently. In view of this, the steepness of a density variation of an edge portion (hereinafter referred to as “edge steepness”) in an image taken is used as a focus measure. Therefore, in auto focusing using a SEM image, it is necessary that an image taken include high-contrast edges. This technique is effective also in SEMs and is also used in review SEMs.
In semiconductor mass-production lines, it is necessary to correctly monitor how defects are occurring in a manufacturing process. To this end, it is necessary that as many wafers as possible be subjected to inspection by an inspection apparatus and observation and classification of defects by a review SEM. In the inspection apparatus and the review SEM, increase in processing speed (i.e., throughput) is particularly important. Conventional techniques relating to such a review SEM are disclosed in JP-A-2001-331784. This reference discloses a configuration of a review SEM, ADR and ADC functions and operation sequences, a method for displaying acquired images and a classification result, and other things.
Performing auto focusing in acquiring a SEM image is disclosed in JP-A-2005-285746.
JP-A-2003-98114 discloses an ADR sequence for determining a defect position without using a reference image by utilizing the periodicity of patterns in an imaging region of a memory cell area.
Furthermore, JP-A-2007-40910 discloses an ADR sequence for determining a defect position without using a reference image even in the case where an imaging region includes part of logic patterns that exist in a peripheral portion of a memory cell area.
FIG. 2 shows a conventional ADR sequence. In general, errors of defect coordinates that are output from a defect inspection apparatus with respect to actual defect coordinates have variations of about ±4 μm. Therefore, when a region that has a field of view of about 2.5 μm and should include defect coordinates that are output from a defect inspection apparatus is taken at a high magnification (e.g., 50,000), the defect may not be included in the field of view. This is avoided in the following manner. First, a region having a field of view of about 9 μm is imaged at a low magnification (e.g., 15,000) (hereinafter referred to as “low-magnification imaging”). Defect coordinates are determined from a resulting low-magnification image. Finally, a position corresponding to the determined defect coordinates is imaged at a high magnification (hereinafter referred to as “high-magnification imaging”).
First, at step S201, to acquire a low-magnification reference image, a table that is mounted with a specimen is moved to a reference coordinates position. At step S203, a coordinates position that is free of a defect and has the same wiring patterns as a defect coordinates position is imaged at a low magnification (taking of a low-magnification reference image). At step S204, to acquire a low-magnification defect image, the stage is moved to the defect coordinates position. At step S206, the defect coordinates position is imaged at the same low magnification (taking of a low-magnification defect image). The reference image and the defect image are taken after calculating target focusing positions of the two imaging regions by auto focusing and setting the focusing position to the target focusing positions (S202 and S205), respectively. At step S207, defect coordinates are determined by taking a difference between the two images acquired.
In general, the depth of focus is shallower in high-magnification imaging than in low-magnification imaging. Therefore, a target focusing position needs to be determined with higher accuracy in high-magnification imaging. In SEM image auto focusing, when the depth of focus is great, it is difficult to determine a target focusing position accurately. Therefore, the accuracy of a target focusing position obtained by low-magnification SEM image auto focusing is insufficient for high-magnification imaging. In view of this, in high-magnification imaging, at step S209 SEM image auto focusing is performed again at a high magnification. As a result, the time taken by auto focusing accounts for a large part of the time taken by a defect review, which is a factor of throughput reduction.
In SEM image auto focusing, it is necessary to image a region where high-contrast edges exist (hereinafter referred to as “edge region”) such as a region where wiring patterns are formed. In JP-A-2005-285746, the imaging time and the processing time are shortened by extracting a narrow region including edges from a low-magnification image and setting it as an imaging region of high-magnification SEM image auto focusing (hereinafter referred to as “auto focus execution region”) (S208).
Where the technique disclosed in JP-A-2005-285746 is used in ADR for observing a defect detected by a separate inspection apparatus, since an auto focus execution region is set after low-magnification imaging, it is difficult to increase the processing speed of low-magnification SEM image auto focusing which is performed before the low-magnification imaging. That is, in the ADR sequence of the conventional method, since low-magnification imaging is performed in a state that an auto focus execution region has not been set yet, a target focusing position cannot be detected reliably by one auto focusing operation. There may occur a case that a region suitable for auto focusing needs to be found by repeating auto focusing. The time taken by low-magnification SEM image auto focusing accounts for a large part of the ADR processing time. It is therefore desired to shorten the processing time of low-magnification SEM image auto focusing.
In a region on a semiconductor wafer where no wiring patterns are formed, edges exist only in a defect. In such a case, it is difficult in terms of principle to determine a target focusing position with high reliability by low-magnification SEM image auto focusing because an imaged defect is small and hence a focus measure cannot be calculated stably. The conventional ADR sequence has a problem that it is unstable because a target focusing position is calculated by low-magnification SEM image auto focusing even in regions where no wiring patterns are formed.